This invention relates generally to magnetic resonance imaging, and more particularly the invention relates to spectrally selective steady-state free precession (SSFP) imaging using magnetic resonance imaging.
Magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse traverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation, the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
Referring to the drawings, FIG. 1A is a perspective view partially in section illustrating coil apparatus in an NMR imaging system, and FIGS. 1B-1D illustrate field gradients which can be produced in the apparatus of FIG. 1A. This apparatus is discussed by Hinshaw and Lent. "An Introduction to NMR Imaging: From the Bloch Equation to the Imaging Equation." Proceedings of the IEEE, Vol. 71, No. 3, March, 198, pp. 338-350. Briefly, the uniform static field B.sub.0 is generated by the magnet comprising the coil pair 10. A gradient field G (x) is generated by a complex gradient coil set which can be wound on the cylinder 12. An RF field B.sub.1 is generated by a saddle coil 14. A patient undergoing imaging wold be positioned along the Z axis within the saddle coil 14.
In FIG. 1B an X gradient field is shown which is parallel to the static field B.sub.0 and varies linearly with distance along the X axis but does no vary with distance along the Y or Z axes. FIGS. 1C and 1D are similar representation of the Y gradient and Z gradient fields, respectively.
FIG. 2 is a functional block diagram of the imaging apparatus as disclose in NMR-A Perspective in Imaging, General Electric Company, 1982. A computer 20 is programmed to control the operation of the NMR apparatus and process FID signals detected therefrom. The gradient field is energized by a gradient amplifier 22, and the RF coils 26. After the selected nuclei have been flipped, the RF coils 26 are employed to detect the FID signal which is passed to the receiver 28 and thence through digitizer 30 for processing by computer 20.
Magnetic resonance imaging is a major noninvasive method of medical diagnosis. However, two limitations against extending clinical applicability are cost and diagnostic sensitivity. If scan time is reduced without sacrificing SNR or contrast (sensitivity), the technique becomes more cost effective. Steady state free precession (SSFP) methods permit fast imaging with increased signal, but suffer from banding artifacts due to B.sub.0 inhomogeneity. Additionally, SSFP techniques yield an undesirably intense lipid proton signal.
In accordance with the present invention, we present two related solutions addressing these issues. In one solution called fluctuating equilibrium magnetic resonance (FEMR); a novel pulse sequence is presented that produces a magnetization that fluctuates from excitation to excitation in the steady state, thus permitting the simultaneous acquisition of multiple images with differing contrast.
In accordance with the other solution to these two shortcomings of SSFP imaging, a novel postprocessing of linear combinations of Fourier data from several SSFP sequences yielding several images upon reconstruction, each with a different contrast. We call this method linear combination SSFP (LCSSFP).